The origin of grainy dolomite and crystalline dolomite as the most important natural gas reservoir rocks in the Triassic Feixianguan Formation, Northeastern Sichuan Basin, is always one of the hot research focuses to the sedimentologist and petroleum geologist. Based on the formation temperatures of different types of dolomites in the Triassic Feixianguan Formation, Northeastern Sichuan Basin, and the relationship between the temperature and Ca/Mg ratio of the dolomitizing fluid, this paper estimated quantitatively the probably main distribution of Ca/Mg ratio of the dolomitizing fluids in the Triassic Feixianguan Formation, Northeastern Sichuan Basin. The results indicated that the Ca/Mg ratios of the dolomitizing fluids corresponding different types of dolomites differ from each other, and also confirmed that the Ca/Mg ratios of the Triassic seawater or related fluids, mixing-fluids with Triassic seawater and meteoric water which ranged from 40℃ to 130℃ or even higher temperatures meet to the requirement of the dolomitization (dolomite precipitation). The micrite dolomite and grainy dolomite (with original texture) could be related to relatively rapid precipitation at low temperature from the surface or near-surface mixing-fluids with small Ca/Mg ratio, and these dolomites are relatively fine and anhedral. The crystalline dolomite (including the transitional rock types with the similar texture and dolomite cement) could be related to relatively slow precipitation at high temperature from the burial Triassic seawater or related fluids with large Ca/Mg ratio, and these dolomites are relatively coarse and euhedral. The quantitative Ca/Mg ratio estimation of the dolomitizing fluids will provide some valuable basic data to the studies of the high-quality dolomite reservoirs in the Triassic Feixianguan Formation, Northastern Sichuan Basin.